Thernal spraying, also known as flame spraying, involves the heat softening of a heat fusible material such as metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (149 microns) and about 2 microns.
A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion thermal spray gun, the carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon. Hydrogen or helium is usually added to the primary gas. The carrier gas is generally the same as the primary plasma gas, although other gases, such as hydrocarbons, may be used in certain situations.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized, usually by blast gas, and then propelled in finely divided form onto the surface to be coated. In an arc wire gun two wires are melted in an electric arc struck between the wire ends, and the molten metal is atomized by compressed gas, usually air, and sprayed to a workpiece to be coated, the rod or wire may be conventionally formed as by drawing, or may be formed by sintering together a powder, or by bonding together the powder by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the powder to be sprayed in finely divided form.
A class of materials known as hard facing alloys are used for coatings produced, for example, by thermal spraying. Such alloys of iron contain boron and silicon which act as fluxing agents during processing and hardening agents in the coatings. Generally the alloy coatings are used for hard surfacing to provide wear resistance, particularly where a good surface finish is required.
An iron alloy for surfacing may contain chromium, boron, silicon and carbon, and may additionally contain molybdenum and/or tungsten. For example U.S. Pat. No. 4,064,608 discloses iron-base hardfacing alloys that range in composition from (in weight percentages) about 0.5 to 3% Si, about 1 to 3% B, 0 to 3% C, about 5 to 25% Cr, 0 to 15% Mo, 0 to 15% W and the balance essentially iron. This alloy is indicated therein for application on yankee drier rolls for the processing of paper, involving wet, corrosive conditions at elevated temperature. This alloy is not as good as may be desired with respect to acid corrosion and frictional wear.
In certain instances copper is incorporated in a molybdenum-containing alloy. U.S. Pat. No. 4,536,232 describes a cast iron alloy of (in weight percentages) about 1.2 to 2 carbon, 1-4 nickel, 1-4 molybdenum, 24-32 chromium, up to 1 copper and up to about 1% of a microalloying element that may include boron.
A similar group of iron alloys may exist in an amorphous form. They contain such elements as molybdenum and/or tungsten, and boron, silicon and/or carbon. The alloys are prepared with the amorphous structure by rapid quenching from the melt. For example amorphous ribbon may be produced by quenching a stream of molten alloy on a chilled surface as described in U.S. Pat. No. 4,116,682. A practical method of processing such alloys into a directly useful form is by thermal spraying to produce a coating.
Aforementioned U.S. Pat. No. 4,116,682 describes a class of amorphous metal alloys of the formula MaTbXc wherein M may be iron, cobalt, nickel and/or chromium; T may include molybdenum and tungsten; and X may include boron and carbon. The latter group X of boron, etc. has a maximum of 10 atomic percent which calculates to about 1.9% by weight for boron in the amorphous alloys; thus boron is characteristically low compared to the boron content in the ordinary hardfacing alloys.
An amorphous iron based alloy directed to fatigue property is disclosed in U.S. Pat. No. 4,473,401, containing, in atomic percent: 25% or less of Si; 2.5 to 25% of B, providing that the sum of Si and B falls in the range of 17.5 to 35%; 1.5 to 20% of Cr; 0.2 to 10% of P and/or C; 30% or less of at least one element of a group of twelve that includes Mo and Cu; balance Fe; with effective maximums given as 5% for Mo and 2.5% for Cu. In converted units the maximum for copper is about 0.8% by weight. Alloys of this type are limited in wear resistance and acid corrosion resistance.
The iron based compositions are of interest for their low cost compared to nickel and cobalt alloys. However, for the combined properties of corrosion resistance, frictional wear resistance and abrasive wear resistance, further improvements in these properties are desired.
In view of the foregoing, a primary object of the present invention is to provide a novel iron alloy composition characterized by the combination of corrosion resistance, frictional wear resistance and abrasive wear resistance.
A further object of this invention is to provide an improved amorphous type of alloy for the thermal spray process.
Another object is to provide an improved thermal spray process for producing corrosion and wear resistant coatings.